Anatomy

Nervous System

CNS, PNS, autonomic, somatic — the body's electrochemical signaling network

The nervous system is the body's command-and-control network, organized hierarchically. Central nervous system (CNS) — brain (~86 billion neurons; ~1.4 kg) and spinal cord. Peripheral nervous system (PNS) — 12 cranial nerve pairs and 31 spinal nerve pairs leaving the CNS. Functionally divided into somatic (voluntary, skeletal muscle, conscious sensation) and autonomic (involuntary; sympathetic "fight or flight" and parasympathetic "rest and digest"; enteric — the gut's own ~500 million neuron mini-brain). Communication uses electrical signals along axons (action potentials at 0.5-120 m/s) and chemical signals at synapses (neurotransmitters acting on receptors). Disease spans stroke (#2 cause of death globally), neurodegeneration (Alzheimer, Parkinson), MS, neuropathies, psychiatric illness — ~1 in 3 adults will have a neurological disorder in their lifetime.

  • Brain neurons~86 billion (Herculano-Houzel, 2009)
  • Brain mass~1.4 kg adult (~2% body weight)
  • Brain energy use~20% of resting metabolism
  • Cranial nerves12 pairs
  • Spinal nerves31 pairs
  • Action potential conduction0.5 m/s (unmyelinated C) to 120 m/s (Aα myelinated)

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Why the nervous system matters

  • Cognition and behavior. Brain seat of thought, memory, emotion.
  • Sensation. Vision, hearing, touch, pain, balance.
  • Motor control. Voluntary movement and reflexes.
  • Homeostasis. Autonomic regulation of HR, BP, GI, temperature.
  • Disease burden. Stroke, dementia, depression, neuropathy.
  • Pharmacology. Most psychiatric and neurologic drugs target receptors here.
  • Surgery. Neurosurgery, anesthesia, regional blocks all rely on neural anatomy.

Common misconceptions

  • We use only 10% of our brain. Imaging shows essentially all regions are active over time.
  • Left brain logical, right brain creative. Hemispheres specialize but most tasks engage both.
  • Brain cells never regenerate. Adult neurogenesis exists in hippocampus and olfactory bulb (limited).
  • Sympathetic only acts in emergencies. Tonically active — maintains BP, heart rate baseline.
  • CNS and PNS heal similarly. PNS regenerates ~1 mm/day; CNS axons largely don't.
  • Pain comes from injured tissue alone. Brain processing modulates — chronic pain has central components.

Frequently asked questions

What's the difference between CNS and PNS?

CNS — brain and spinal cord, encased in bone (cranium and vertebrae) and bathed in cerebrospinal fluid; protected by blood-brain barrier. Glial cells include astrocytes, oligodendrocytes (myelinate CNS axons), microglia, ependymal cells. PNS — everything outside; cranial nerves (CN I-XII), spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal), ganglia, peripheral nerves. Schwann cells myelinate. Different injury responses: PNS axons regenerate (~1 mm/day after Wallerian degeneration); CNS axons largely don't due to inhibitory environment (Nogo, MAG, OMgp).

How does the autonomic nervous system work?

Two-neuron chain (preganglionic and postganglionic). Sympathetic — "thoracolumbar" (T1-L2 origin); short pre, long post; ganglia in sympathetic chain or prevertebral; pre uses ACh on nicotinic, post uses NE on α and β adrenergic; mass response (fight or flight). Parasympathetic — "craniosacral" (CN III, VII, IX, X; S2-S4); long pre, short post; ganglia near or in target organ; both pre and post use ACh (nicotinic on ganglia, muscarinic on effectors); discrete responses. Enteric — gut neurons; can function autonomously.

What does each cranial nerve do?

I Olfactory (smell). II Optic (vision). III Oculomotor (most eye muscles, lid, pupil). IV Trochlear (superior oblique). V Trigeminal (face sensation, mastication). VI Abducens (lateral rectus). VII Facial (face muscles, taste anterior 2/3 tongue, lacrimation). VIII Vestibulocochlear (hearing, balance). IX Glossopharyngeal (post 1/3 tongue taste, swallow). X Vagus (parasympathetic to thoracoabdominal, voice). XI Accessory (SCM, trapezius). XII Hypoglossal (tongue movement). Mnemonic: "Some Say Marry Money But My Brother Says Big Brains Matter More" for sensory/motor/both.

What is the spinal cord?

~45 cm long; ends at L1-L2 vertebra (conus medullaris); cauda equina continues lumbosacral roots. Gray matter (cell bodies) butterfly-shaped centrally — dorsal horn (sensory), ventral horn (motor), lateral horn (autonomic, T1-L2). White matter (myelinated tracts) — ascending sensory (dorsal columns for fine touch/proprioception, spinothalamic for pain/temperature) and descending motor (corticospinal). Lumbar puncture done below conus, typically L3-L4 or L4-L5 (Tuffier's line). Brown-Sequard syndrome — hemicord lesion gives ipsilateral motor and proprioception loss, contralateral pain/temperature loss.

How does a stroke present?

Sudden focal neurologic deficit due to interrupted blood flow. Ischemic (~85%): thrombotic, embolic, lacunar. Hemorrhagic (~15%): intracerebral, subarachnoid. Pattern depends on territory. MCA: contralateral face and arm weakness, aphasia (dominant), neglect (non-dominant). ACA: contralateral leg weakness. PCA: homonymous hemianopia. Brainstem: crossed signs, ocular motor abnormalities. FAST mnemonic for public — Face droop, Arm weakness, Speech change, Time. Time = brain (~1.9 million neurons lost per minute). tPA within 4.5 hr; thrombectomy up to 24 hr in selected patients.

What's the blood-brain barrier?

Tight junctions between brain capillary endothelial cells; supported by astrocyte foot processes and pericytes. Excludes most large or polar molecules. Lipid-soluble drugs (alcohol, anesthetics, many psychotropics) cross freely. Active transporters carry glucose (GLUT1), amino acids, some drugs (and pump out via P-glycoprotein). Drug delivery challenge for chemotherapy and biologics. Disrupted in stroke, infection, MS, tumor (gadolinium enhancement on MRI signals breakdown). Circumventricular organs (area postrema, posterior pituitary) lack BBB — sense blood and secrete hormones.

What about meninges and CSF?

Three layers cover brain and cord: dura (tough outer), arachnoid (middle), pia (delicate, on surface). Subdural space (potential, between dura and arachnoid — bleeds in elderly with shrinking brains, bridging veins tear); subarachnoid space (where CSF flows, contains arteries — ruptured aneurysm here causes "thunderclap" headache). CSF — produced by choroid plexus (~500 mL/day; ~150 mL volume circulating); cushions brain, removes waste, ionic homeostasis. Reabsorbed via arachnoid granulations and lymphatics (recently characterized meningeal lymphatic system). Hydrocephalus — obstructed flow or impaired absorption.